DESCRIPTION: This laboratory's long-term research goal is to define, at the molecular level, the network of pathways that function together to maintain genome integrity and stability in eukaryotic cells. These pathways play a critical role in protecting against genetic changes that can occur as a consequence of DNA damage by environmental agents. In somatic cells the mutations may then serve as the initiating agent for the multistep pathway that leads to metastatic cancer, whereas in germ cells the mutations may cause fetal inviability or birth defects. DNA joining is an essential step in the replication, excision repair and recombination of DNA molecules. Three mammalian genes encoding DNA ligases, LIG1, LIG3 and LIG4, have been identified. It is hypothesized that each of these gene products has a distinct function in mammalian DNA metabolism. Although there is substantial biochemical and molecular genetic data demonstrating that DNA ligase I is required for DNA replication and also participates in DNA repair, the cellular roles of the LIG3 and LIG4 gene products are less well understood. To address this, an investigation is being undertaken of the multiplicity of DNA ligases in Saccharomyces cerevisiae because of the relative ease of genetic analysis in this model eukaryotic organism. A novel DNA ligase activity and a yeast open reading frame, YOR005c, that exhibits homology with the mammalian LIG4 have been identified. In parallel the mammalian LIG3 and LIG4 genes will be studied. Since both these genes are highly expressed in the testis, their involvement in the processes of replication, recombination and repair that occur during male germ cell differentiation will be examined. The subcellular distribution of the gene products in somatic and germ cells will be compared, and proteins expressed in the testis that interact with DNA ligase III or DNA ligase IV will be identified by biochemical and genetic approaches. In summary, the goal of these studies on eukaryotic DNA ligases is to provide fundamental information about DNA repair and recombination processes that protect the genome in both somatic and germ cells from the potentially deleterious effects of environmental DNA damaging agents.